The impact of ambient pressure on the gas temperature near the ceiling and air entrainment caused by two fires in a tunnel

The tunnel is a typical underground building. Estimating the heat released by the fire sources to the surrounding environment, as well as providing rapid emergency response, requires a thorough comprehension of the properties of gas temperature and air entrainment beneath tunnel ceilings. Numerical works are performed to explore the impact of pressure on the gas distribution, velocity field distribution, temperature field distribution, and air entrainment rate induced by dual interacting fires with various heat release rates in a tunnel building. The findings indicate that as the distance between fires increases, two flames undergo a transition from merging together to not merging (S/D >= 2). The rise in atmospheric pressure leads to an augmentation in the air intake of two fires, ultimately diminishing the intensity at which the flames merge. As the spacing or pressure decreases, the area of flame expansion on the tunnel ceiling grows. The oxygen level in the extra space between the two flames gradually rises as the ambient pressure rises. This leads to a reduction in the local gas speed and temperature. The constraint factor is utilized to describe the maximum strength of air entrainment, revealing that the air entrainment constraint factor (beta) for two tunnel fires ranges from 0 to 0.2. The maximum temperature rise of ceiling gas (Delta T-max) is reached at zero fire spacing, which is about 900 degree celsius. For two fires with large spacings (S/D >= 2), Delta T-max is mainly determined by ambient pressure and heat release rate. Given that less cryogenic air entrainment into the ceiling gas, a higher Delta T-max is achieved at lower ambient pressure. A segmented correlation to correlate Delta T-max of two fires is developed and well-verified using literature data, and the forecast error is within +/- 30%.